1. Field of the Invention
The present invention relates to a collimator for an X-ray examination apparatus having several diaphragm plates adjustable in pairs relative to one another for defining a radiation cone having a rectangular cross-section of variable size, and in particular to such a collimator having an additional set of diaphragm plates pivotable at the four corner regions of the radiation cone for defining an approximately circular radiation cone of variable diameter.
2. Description of the Prior Art
A collimator for X-rays is disclosed in U.S. Pat. No. 2,675,486 wherein four diaphragm plates are mounted in a displaceable manner in a plane disposed perpendicularly to the direction of radiation propagation. This conventional collimator generates a rectangular radiation cone of variable size. If used in combination with conventional X-ray image intensifiers, which have a round inlet fluorescent screen, a 30% overshoot of radiation at the corners of the rectangular radiation cone generated by such conventional collimators is necessary in order to obtain full illumination of the field of the inlet fluorescent screen. Such radiation overshoot decreases the contrast of the resulting radiograph and increases the radiaton exposure of the patient.
Another collimator is disclosed in German As No. 1037035 for use in radiation therapy wherein four diaphragm plates are also displaceably mounted in a plane perpendicular to the direction of radiation propagation, each of the diaphragm plates being obliquely subdivided. Each diaphragm plate thus consists of two approximately triangular halves which are displaceable relative to each other by means of a spindle. This collimator which permits definition of tetragonal (square) to maximally octagonal fields, is quite costly to manufacture and moreover requires independent adjustment of five control grips in order to operate.
A collimator is described in German Pat. No. 2053089 for an X-ray examination apparatus which again has a set of diaphragm plates for defining a rectangular radiation cone and has an additional set of a diaphragm plates disposed in a different plane, those plates being triangular and having edges which slide against each other. The additional plates are adjustable along a curved guidance means about the axis of symmetry of the collimator. Depending upon the number of additional triangular diaphragm plates which are utilized, this collimator permits cross sections which approximate a circle to be defined. This collimator, however, has the disadvantage that the large number of edges of the plates sliding against each other, with substantially no gap therebetween, requires a considerable amount of friction to be overcome during adjustment. This disadvantage makes this collimator generally unsuitable for remote control. Another disadvantage is that the collimator must be regularly serviced in order to avoid jamming of the triangular diaphragm plates.
Another collimator which permits definition of a rectangular radiation cone is distributed by the Philips Company. This collimator reduces radiation overshoot of the circular inlet fluorescent screen by the use of additional diaphragm plates disposed in at least two different planes which can be pivoted into the radiation cone. The additional diaphragm plates must be arranged in at least two different planes so as to avoid obstructing each other in the case of small radiation fields. Moreover, the radiation overshoot of the round inlet fluorescent screen is dependent upon the diameter of the defined radiation cone.